1. Field of the Invention
The present invention relates to a water-decomposable fibrous sheet capable of being readily decomposed and dispersed in water flow. More precisely, it relates to such a water-decomposable fibrous sheet having good decomposability in water and high strength in wet.
2. Description of the Related Art
To wipe the skin of human bodies including the private parts thereof, or to clean toilets and thereabouts, used are disposable cleaning sheets made of paper or non-woven fabric. For these cleaning sheets, water-decomposable cleaning sheets that could be directly disposed of in toilets after use have been developed, as being convenient for such purposes. The degree of their decomposability in water must be high in some degree. This is because, if poorly water-decomposable cleaning sheets are disposed of in toilets after use, they will take a lot of time until they are decomposed and dispersed in septic tanks, or will clog the drainpipes around toilets, etc.
For wiping off wet dirt and for easy and effective use, many cleaning sheets for wiper applications are packaged while being wetted with a liquid detergent chemical or the like, and are put on the market. Therefore, such water-decomposable cleaning sheets must have high strength in wet to such a degree that they are well fit for wiping with them wetted with such a liquid chemical or the like, but must well decompose in water after they are disposed of in toilets.
For example, Japanese Patent Publication No.24636/1995 discloses a water-decomposable cleaning article that comprises a water-soluble binder having a carboxyl group, a metal ion and an organic solvent. However, the metal ion and the organic solvent irritate the skin.
Japanese Patent Laid-Open No. 292924/1991 discloses a water-decomposable cleaning article of polyvinyl alcohol-containing fibers with an aqueous solution of boric acid infiltrated thereinto; and Japanese Patent Laid-Open No. 198778/1994 discloses a water-decomposable napkin of polyvinyl alcohol-containing non-woven fabric with a borate ion and a bicarbonate ion introduced thereinto. However, polyvinyl alcohol is not resistant to heat, and therefore the wet strength of the water-decomposable cleaning article and the water-decomposable napkin is lowered at 40xc2x0 C. or higher.
Recently, various water-decomposable absorbent articles including sanitary napkins, panty liners, disposable diapers and others have been investigated in the art. In view of their safety, however, the water-decomposable fibrous sheets mentioned above could not be used as the top sheets for those absorbent articles that-shall be kept in direct contact with the skin for a long period of time, as they contain a binder and an electrolyte.
On the other hand, Japanese Patent Laid-Open No. 228214/1997 discloses a water-degradable non-woven fabric having a wet strength of from 100 to 800 gf/25 mm (from 0.98 to 7.84 N/25 mm) as measured according to JIS P-8135, which is produced by mixing fibers having a length of from 4 to 20 mm with pulp followed by entangling them through treatment with high-pressure water jets. Since the constituent fibers are entangled in it, the non-woven fabric disclosed has a bulky feel. However, in producing the non-woven fabric, long fibers are entangled through high-pressure water jet treatment, whereby the non-woven fabric produced could have such a relatively high wet strength. Therefore, according to the technique disclosed, it is difficult to realize well-balanced bulkiness, strength and water-degradability for the non-woven fabric produced, and the non-woven fabric produced is unsuitable to disposal in flush toilets, etc.
An object of the invention is to provide a water-decomposable fibrous sheet which is well decomposed in water and has good strength enough for wiper applications even though no binder is added thereto.
Another object of the invention is to provide a water-decomposable fibrous sheet which is safe for its application to the skin.
According to one aspect of the invention, a water-decomposable fibrous sheet may comprise fibers containing fibrillated rayon, the fibrillated rayon having primary fibers of a predetermined fiber length and microfibers extending from the primary fibers;
the fibrillated rayon including a first type of fibrillated rayon having a degree of beating of at most 700 cc, of which the length of the primary fibers falls between 1.8 mm and 4.0 mm at the peak of its self-weighted, average fiber length distribution profile curve, and a second type of fibrillated rayon having a degree of beating of at most 700 cc, of which the length of the primary fibers falls between 4.5 mm and 10.0 mm at the peak of its self-weighted, average fiber length distribution profile curve, and
the microfibers extending from the first and second types of fibrillated rayon being entangled with and/or hydrogen-bonded to at least either of other microfibers and other fibers.
Naturally in dry and even in wet with water, the water-decomposable fibrous sheet of the invention all the time keeps high strength. When it is immersed in a large amount of water after used and disposed of in toilets and others, it is readily decomposed. In the fibrous sheet of the invention, the microfibers of fibrillated rayon are entangled with and are further hydrogen-bonded to other fibers and other microfibers therein, thereby exhibiting their ability to bond fibers constituting the sheet and to enhance the strength of the sheet. When the fibrous sheet receives a large amount of water applied thereto, the entangled microfibers therein are loosened or the hydrogen bonds between the bonded microfibers therein are broken, whereby the fibrous sheet is readily decomposed in water. In particular, in the fibrous sheet of the invention, used are the first type of fibrillated rayon of short fibers and the second type of fibrillated rayon of long fibers. Therefore, the fibrous sheet realizes well-balanced decomposability in water, dry strength and wet strength.
The water-decomposable fibrous sheet of the invention can be composed of materials not harmful to human bodies.
In the fibrous sheet, preferably, the microfibers having a length of at most 1 mm in the first and second types of fibrillated rayon account for from 0.1 to 65% by mass of the self-weight of the fibrillated rayon.
The fibrous sheet may contain each of the first and second types of fibrillated rayon in an amount of at least 3% by mass of all the fibers constituting the fibrous sheet. Preferably, it contains at least 5% by mass of other fibers having a length of at most 10 mm, in addition to the fibrillated rayon.
The fibrous sheet may be a non-woven fabric having been subjected to water-jetting treatment. Alternatively, it may be produced in a paper-making process.
Preferably, the degree of fineness of the fibrillated rayon falls between 1.1 and 1.9 dtex.
Also preferably, the weight (this may be referred to as xe2x80x9cMetsukexe2x80x9d) of the fibers constituting the fibrous sheet falls between 20 and 100 g/m2.
Still preferably, the decomposability in water of the fibrous sheet, measured according to JIS P-4501, is at most 200 seconds.
Still preferably, the wet strength of the fibrous sheet is at least 1.1 N/25 mm.
Also preferably, the dry strength of the fibrous sheet is at least 3.4 N/25 mm.